"Tokens" are a security measure we had to implement along the way to prevent people from "faking" pulses with double statistics. A token is generated every time you log in and when you pulse your statistics. This token is stored locally on your computer to authenticate with to the server next time the client pulses.
It sometimes happens that your computer fails to save this token properly, like due to a systems crash. In that case, you can get stuck with an "Invalid Token" message when you try to pulse.
The only way to solve that and continue pulsing is to reset your token. You can find that option on the "Account" tab - there is an button called "Reset Token" - click that and you will be good to go! This action will reset only your unpulsed statistics, not your totals or heat maps.